Dovetail attachment techniques between turbine buckets and turbine rotor wheels for steam turbines are well known in the art. Conventional tangential entry dovetails on the latter stages of low-pressure rotors operating in a contaminated steam environment have been found to be conducive to stress corrosion cracking (SCC). SCC is accelerated by the stress levels that are present in the hook fillet regions of typical dovetail configurations. Normally, these stresses are acceptable but with contaminated steam, cracks can initiate and, if left undetected, may grow to a depth that will cause failure of the wheel hooks. In extreme cases, all the hooks will fail and buckets will fly loose from the rotor. Long experience with bucket-to-wheel dovetail joints has indicated that the wheel hooks crack but that the bucket hooks do not crack. This is apparently because the NiCrMoV and similar low-alloy steels used for low-pressure rotors are much less resistant to SCC than are the 12Cr steels used for buckets. The steels for the wheels give the optimum combination of properties available for overall low-pressure rotor design considerations. Thus, an effective means of avoiding SCC in the typical low-pressure steam environment is to reduce the stresses in the wheel dovetail to acceptable levels. If the maximum stress in components operating in a corrosive environment is below the yield strength of the material, the resistance to SCC is greatly improved.
One bucket and wheel dovetail design for steam turbine rotors has been described and illustrated in U.S. Pat. No. 5,474,423, of common assignee. In that patent, the dovetail joint design provided four hooks on the rotor wheel which decreased in thickness from the radially outermost hooks to the innermost hooks. Additionally, fillets were provided between neck portions of the rotor wheel dovetails and bottom surfaces of the overlying hooks, with multiple radii, i.e., compound fillets, in order to decrease the stress concentrations with increased radii of the fillets. Additional features of that design included a flat surface along the radially outermost surface of the hook and in combination with various forms of compound fillets. While that dovetail design has been eminently satisfactory, the present dovetail joint provides a configuration which also minimizes concentrated stresses in the wheel hook fillets, while maintaining an overall size compatible with existing steam paths.